This mat is composed of an assemblage of organisms including cyanobacteria and diatoms which trap and bind the sediment. Microbial communities similar to the benthic communities found in the McMurdo Dry Valley lakes were the dominate life forms on Earth until about 600 million years ago. These ancient microbial communities left evidence of their existance in the fossil record in the form of structures known to geologists as stromatolites. The earliest stromatolite fossils have been dated to about 3.5 billion years ago. In the McMurdo Dry Valley lakes there are no higher organisms such as vertebrates; all life found in these very unique lakes is microbial.